On a pleasant spring day in the final week of April, the foundations were laid for the fourth green roof at Swarthmore College. Whilst merry old England was celebrating the just completed nuptials of the Duke and Duchess of Cambridge (Will and Kate), gardeners at the Scott Arboretum were mobilizing for the arrival of some heavy equipment. A large crane was carefully maneuvered into a small parking lot and immediately employed to deliver pallets of materials skyward to the long anticipated Wister Center green roof.

The Wister Center green roof has earned a special place in the hearts of arboretum staff because it is the first one we have installed (almost) all by ourselves. Previously the college has employed outside contractors to design and install our green roofs, and we gardeners have served as enthusiastic acolytes and willing apprentices, as well as loyal maintainers. This time, however, we came close to flying solo.

Our co-pilot was Peter Philippi the seasoned green roof guru who designed our Alice Paul and David Kemp roofs. Peter has direct experience with many vegetated roofs including a thirty-year-old green roof in Germany that is still going strong.

Most of our materials needed to come via pallets hooked to the end of the very long arm of a crane. Local arborist Knight Brothers very generously donated the services of this crane and its operator.

On top of the roof’s waterproof membrane goes a thick blanket of recycled polyester fabric that both protects the waterproof membrane and holds on to moisture.

Next came a drainage layer: thin, stiff plastic sheets with drainage channels molded into them. This was a change from our earlier green roofs, which, for the drainage layer, had used an expanded shale product called Solite that was developed and generously donated by an alumnus of the college.

The plastic drainage sheets were used because, unlike our other roofs, the Wister Center has sections of pitched roof that drain onto the green roof. We anticipate these pre-molded drainage channels will be more efficient in handling the increased volume and velocity of run-off from these pitched roofs.

On top of the drainage layer is a second, thinner blanket of recycled polyester, acting as a filter to keep particles of growing media from clogging the drainage layer and to retain moisture.